For testing purposes you can tell the application to somewhat ignore compatibility checks when installing add-ons. Simply create the boolean preference extensions.checkCompatibility and set it to false.
RESTTest allows you to construct custom HTTP requests to directly test requests against a server. RESTTest uses XHR object. It is designed specifically for working with REST sources, supporting all HTTP methods, and allows you to issue HTTP requests.
Selenium IDE is an integrated development environment for Selenium tests. It is implemented as a Firefox extension, and allows you to record, edit, and debug tests.